This post was prompted by a newbie posting a question about fix a leak in a compression fitting he'd installed. He installed the compression fitting cuz he didn't know how to solder. And, he'd tightened that compression fitting so much he was scared that further tightening was gonna do more harm than good, but the leak wouldn't stop. So, he couldn't go forward or backward.

I'm just gonna explain the basics of what you do here because what to do is the easiest part of it. It's the WHY you're doing it that I'm probably gonna spend more time on because that's something that not everyone knows. And if anyone has any questions, please post cuz others probably have the same questions. Also, would the experienced plumbers in here PLEASE put in their ideas too. EVERY newbie eventually learns to solder, and in my view, the sooner the better so they don't have so many compression fittings to replace once they start soldering.

1. The Torch: You wanna buy a propane torch that mixes the propane with air in a short tube before burning it. That much is a "must", and all the other bells and whistles like a piezoelectric lighter built into the torch is OK to have, but it's really not a necessity like mixing the fuel and air before igniting it. The famous "bunsen burner" in your high school chemistry classes used this basic principle of mixing the fuel and air before hand to generate much higher temperatures than you can achieve with a "pencil tip" torch (so named because it doesn't mix the air and the fuel before ignition, and the resulting bright blue flame inside a faint blue flame looks like the sharpened tip of a pencil). You want the flame on your torch NOT to have that pencil tip shape inside the flame.
Also, try to get a torch with a curved nozzle so that when pipes are close to a wall (or whatever) you can sneak the torch tip behind the joint you're soldering. That way, by heating the back of the fitting with the flame pointed toward you, and applying the solder to the side of the fitting in front of you, you know that when the solder starts to melt at the front of the fitting, the rest of the fitting that's closer to the flame (which is all of it) is hotter and you'll fill the whole joint with solder. There won't be any cold spots where the molten solder doesn't flow into. Also by appling the flame behind the joint and the solder in front, you can more easily see when the solder starts to melt and better anticipate when the joint is going to start taking solder. So, having a curved tip on your soldering torch is an advantage to getting the torch in the best possible location for successful soldering.

I should mention MAAP gas, I guess.
I think every torch that burns propane can also be mounted on a MAAP gas cannister. MAAP gas stands for methyl-acetylene - propadiene, meaning that MAAP gas is a mixture of two gasses; methyl-acetylene and propadiene.

Acetylene is easy to understand: It's HC|||CH where ||| is meant to represent a TRIPLE carbon-carbon bond. (I just can't make any such thing on my keyboard.) If you replace one of those hydrogens on the end with a methyl group, you have methyl-acetylene, or:

HC|||C-CH3

Propadiene is similar to methyl-acetylene. If you move one of those bonds in the triple bond over to the other side, and balance everything properly, you get propadiene:

H2C=C=CH2

(so, both gasses in MAAP gas have the chemical formula of C3H4, and if truth be known, it would make more sense to call the stuff C3H4 than to call it methyl-acetly-what-ever or MAAP for short.)

It's the breaking of those double and triple carbon-carbon bonds where the lion's share of the heat comes from, so both C3H4 gasses burn hotter than ordinary propane (which is H3C-HCH-CH3). If you burn pure acetylene in pure oxygen (so that you don't waste heat warming up all the nitrogen in the air), you get a flame hot enough to easily melt steel.

So, even though you can use MAAP gas in your propane torch, and many if not most plumbers do do that so they can work faster, I'd discourage it because the fluxes you're using might not tolerate the higher temperatures, and that could lead to some bad solder joints. Put MAAP gas on the back burner until you get proficient at soldering.

I'm gonna post this for the time being cuz my computer's been crashing lately and I don't want to retype it all. Next post: what you do.

The difference is that the oxide copper forms both sticks tenaciously to the underlying copper metal and is highly impermeable to O2 molecules in the air and water (unlike iron rust on both counts). Consequently, as the orange copper metal rusts and forms a brown oxide film over itself, the growing thickness of that brown oxide film better and better protects the underlying copper from further oxidation because of that film's low permeability to O atoms and O2 molecules in the air and water. This is why new copper pennies are orange wheras old copper pennies are brown. Ditto for copper pipe.
And, it's exactly the same reason it doesn't take much more sanding to sand the brown oxide layer off a 10 year old copper pipe as it does to sand it off a 70 year old copper pipe. The two oxide layers are about equal in thickness; not one seven times thicker than the other. And this is why copper lasts darn near forever in water piping. It protects itself from further rusting by forming a highly impermeable layer of rust on itself.

STEP #1: The first step is to remove the brown copper oxide film from the copper pipe and fitting.

It don't matter squat that you can see any brown film on your copper pipe or fitting. The coating of silver atoms they put on low-e glass to make it low-e is only about 70 silver atoms thick, and that layer of 70 atoms is almost completely transparent to visible light. You have to know what to look for to tell ordinary glass from low-e glass. So, a layer of 70 silver atoms is so thin to be almost invisible. Even brand new copper pipe or fittings would have a copper oxide film on them that's at least 70 atoms thick. So, don't expect to see an oxide layer. By the time you can see it, it's VERY thick.

Why PRECISELY do we need to remove that copper oxide layer?
The truthful answer is: I don't know. I don't know if anyone knows.

But, if I had to guess, I'd guess that the answer lies in something called "electronegativity". Metals all share their outer electrons. The atoms of iron and copper and gold exist in a cloud of free electrons. Metals are happy to let their outer electrons wander the neighborhood and associate with kids from the bad side of town.

By contrast, oxygen is highly "electronegative", meaning it hoards it's electrons. Except for Fluorine, Oxygen is the most highly electronegative element there is. So, it's very very different from metals in that regard.Electronegativity - Wikipedia, the free encyclopedia

It's capillary pressure that draws molten solder into the tiny gap between the copper pipe and the socket of the copper fitting. High capillary pressure in turn depends on having low surface tension between a fluid and the walls of the capillary tube, and the greater the similarity between the fluid and the solid, the lower the surface tension and the greater the capillary pressure drawing the fluid into the solid capillary tube. So, to have high capillary pressure drawing molten solder into a soldered joint, having high similarity between the molten solder and the walls of the capillary tube is important.

Because metals are as loose as a drunken sailor with their electrons, and oxygen is the exact opposite, the presence of oxygen in the copper oxide on the surface of the copper pipe or fitting reduces the capillary pressure drawing the molten solder into the solder joint.

It's only a guess, but it's my best guess.

So, step 1 is to remove the copper oxide film from both the outside diameter of the end of the copper pipe AND from the inside diameter of the socket of the copper fitting (since both surfaces will "rust"), and we need to get all the oxide off both surfaces before molten metal will stick to them. And, any way you can do that is fair game.

Typically, you use a piece of sandpaper or a sand "cloth" made for this purpose from a mesh fabric that has abrasives bonded to it's surface to sand the oxide layer off the outer diameter of the pipe end. And you typically use a short round steel bristle brush (made specifically for the purpose) to scratch the oxide layer off the inside diameter of the sockets of the fittings.

When using a "fitting brush" to scratch out the ID of the sockets at the ends of a ball valve, it's a good idea to use a small flat washer to prevent the steel bristles from going any further into the valve than the pipe will. For 1/2 inch ball valves, I find that an ordinary #12 flat washer is the right size to allow the fitting brush to go all the way into the sockets on the ends of the ball valve (save for the thickness of the washer), but not a micrometer to protect the teflon seals of the ball valve from being scratched up.

Also, never believe anyone who tells you to take a ball valve apart before soldering it in. You're gonna create vastly more risk of a leak by taking a ball valve apart (if you can even do that) and putting it back together after soldering than you will soldering it in place as it is. The only thing you need to do is ensure that the valve is in the open position when it's soldered in. I prefer to remove the handle from the valve just to eliminate the risk of burning the plastic coating on the handle.

STEP #2: Preventing that oxide layer from reforming

Since copper "rusts", the copper oxide is gonna start forming again as soon as you sand it off. Consequently, in an ideal world, it'd be best to flux your sanded surfaces as soon as you finish sanding them. And, even in the real world this can often be done if you plan your work in advance.

Flux does a number of things, but probably the most important thing that it does is prevents the bare copper metal from re-oxidizing. That process occurs quiet slowly at room temperature; new orange pennies will take a long time to turn brown since the chemical reaction between copper and oxygen is a relatively slow one. However, at soldering temperatures, that reaction is very rapid and something must be done to prevent the copper metal from forming an oxide film as it's being heated up to the melting temperature of solder.

The normal way of doing that is by coating the bare copper surfaces with FLUX. Flux's primary responsibility is to form a physical barrier between the bare copper metal and the oxygen molecules in the air. As long as oxygen molecules can't come into contact with the bare copper metal, then no oxide film can form on the surface of the bare copper metal.

And, since no one can finish sanding or brushing and apply the flux before even a thin film forms on the bare copper, soldering fluxes will usually contain a salt called zinc chloride.

Zinc chloride acts as an acid at high temperatures, and the unique thing about it is that it dissolves metallic oxides more aggressively than it dissolves the bare metals themselves, and this is especially true at elevated temperatures when the acid is hot. Consequently, the purpose of zinc chloride in soldering fluxes is to dissolve any invitable oxide that forms after you sand or brush, but before you flux. Once you've fluxed, the flux itself will coat the copper and prevent oxygen from coming into contact with it.

So, it's best to flux immediately after removing the copper oxide film, but the zinc cloride is strong enough to allow you a considerable amount of time to fit your plumbing pieces together before fluxing.

Nestor, is this a soldering post or a chemistry class? Simple way is to clean the pipe and the inside of the fitting with sand cloth or proper size brush. Apply flux to both parts. Heat the area of the fitting where you want the solder to travel to. Keep touching the solder to the joint until it melts and starts to flow. Go all the way arround the joint. Remove the heat, Wipe with a gloved finger or rag. After a few minutes, wipe with a damp rag to remove the excess flux, which will turn green if you leave it on the pipe and fitting.

Majakdragon: Yes, that's exactly what every DIY book on plumbing says to do. But, have you ever stopped to ask yourself exactly what that flux actually does when you're soldering a joint? The answer to that quesition will tell you whether or not you actually have to use plumbing flux or if you could use something else instead that also would do the same thing. Not that you'd have any need to use something else, but I think it's important to know what the flux does so that if your joint doesn't take solder, you stand a chance to figure out what went wrong.

Carrying on...

Now, a lot of people consider the Copper Tube Handbook published by the Copper Development Association to be "the bible" of anything to do with copper pipe, including soldering and brazing it.

The Copper Development Association claims in their Copper Tube Handbook that one of the functions of soldering flux is that it IMPROVES the "wettability" of the bare copper to molten solder, but they don't state the relative importance of keeping the copper bare and improving wettability, and this has people thinking that both are equally important. In fact, wettability might make soldering easier for the person doing the soldering, but it really doesn't matter one hoot to the solder joint.

Since it's capillary pressure that draws the molten solder into the joint, you don't need to improve the wettability of the copper metal to molten solder to have high capillary pressure. The two materials are already so similar that you should have plenty of capillary pressure to draw the solder into the joint.

To test that point, I did an experiment which I just repeated today and I invite all of you to do it too. I used ordinary petroleum jelly (which is also called "Vaseline") for flux when soldering a joint in copper piping. Obviously, I applied the Vaseline as quickly as possible after both brushing out the ID of the slip coupling I used, and asap after sanding the end of the pipe.

My thinking was that the Vaseline would only burn at the very small annular ring exposed to air at the end of the solder joint, and inside the pipe at the other end of the solder joint. The petroleum jelly inside the joint would get just as hot, but since there was no air inside the joint, it couldn't burn away and would continue to protect the copper from coming into contact with air despite being awfully hot.

Here are some pictures of the experiment:

This first one is taken immediately after soldering the joint. Note that because the Vaseline close to the end of the joint had burned away and exposed the copper to air, there's no visible solder anywhere around that joint. You can only tell when the joint is full when the solder starts melting and dripping cleanly off the pipe. The molten solder won't sick to the hot pipe at all.

After allowing the joint to cool, I then unsoldered that joint and what I found proved my point.

Here's the end of the pipe at it's worst location. There are some areas where the solder didn't get into, but because they're close to the end of the joint, I attribute this to the Vaseline having burned off in these areas before the solder started melting.

Most of the joint was like this; with the solder coating the entire circumference of the pipe. You can tell that the joint was full because of the beginning of a drip of solder formed on the right side of that pipe. That was the bottom side of the pipe when I unsoldered the slip coupling, and so the still molten solder drained to that side of the pipe.

The solder coated the entire inside of the socket as well. You can see a distinct line where the end of the pipe was in the coupling when it was soldered. The ID of the coupling past the end of the pipe was exposed to air, and the solder wouldn't flow onto that exposed area. But the solder flowed evenly onto the rest of the ID of the coupling. You can see that this joint has enough coverage of solder over it's surface that it would have held up under water pressure.

So, the fact that I could get that solder to flow into and fill the joint by capillary pressure shows that you don't need to improve the wettability of the molten solder to the bare copper to get a good solder joint.

The only conclusion you can draw from this experiment is that the most important thing to get a good solder joint is to keep he air away from the bare copper. That's cuz the Vaseline did nothing EXCEPT that, and I still got a strong solder joint.

But, there's no denying the fact that soldering flux works better than Vaseline when it comes to soldering. Using soldering flux makes soldering easier, but the fact that you can get a proper solder joint just using Vaseline as flux means that the ONLY thing that's critical in soldering is to physically prevent the oxygen in the air from coming into contact with the copper during the whole process, and any way you can do that is fair game.

The pro plumbers in here are invited to try this themselves to prove it to themselves.

If you didn't follow or understand this part, don't worry about it. Just remember that the two steps so far in soldering are to:
1. Sand the pipe ends and brush out the fitting sockets.
2. Flux the sanded or brushed copper metal with soldering flux, and ideally, do that asap after sanding or brushing. And, use store bought flux.

And, the main thing the flux does is to form a physical barrier between the air and the bare copper during the soldering process.

OK, so the basic steps in soldering so far are to remove the copper oxide from the surfaces you want the flux to stick to, and then coat those surfaces with flux.

STEP #3: Applying the heat

It's actually best to turn your torch on full blast when soldering. RAPIDLY heating the joint helps to prevent the surrounding piping from getting too hot, and that will prevent nearby solder joints from coming apart. If you're concerned about two joints being so close together that you can't heat one without melting the other, wrap a damp rag around the joint you want to avoid melting.

One of the reasons for soldered joints to leak is that the joint isn't hot enough. If some areas of the joint are cooler, then the solder can solidify before flowing completely around the joint.

To prevent that from happening, most people will apply the torch to one side of the joint and the solder to the other. The thinking behind this is that the solder will be applied to the coolest part of the joint, so when the solder does start to melt, the rest of the joint will be hotter and so you can be reasonably confident that the solder will fill the whole joint.

STEP #4: Knowing when to stop soldering

When soldering a joint, it's a good idea to keep an eye out for three things; the solder filling the joint and forming a silver ring around where the pipe enters the socket, the bottom of the joint where excess solder would drip out of the joint and the amount of solder you've melted into the joint. I normally bend my solder so that I know about how much to use first, then when soldering I watch the spot where I'm applying the solder to see if I see a silver ring, and if I don't then I glance at the bottom of the joint to see if there's a drop forming. If I don't see either the silver ring or the drop forming, then I might go a bit past the bend in my solder, but not by much. I'll try to avoid the urge to keep adding solder until you see one of those two things.

If and when you see the solder fill the joint, it'll appear as a silver ring that seems to go around the circumference of the joint almost instantaneously. Once you see that, then the joint is full and you don't need to apply any more solder. However, as the solder flows into the joint, the flux that was in the joint gets pushed out, and as it comes into contact with the air it burns and forms a dark brown "tar" that might prevent you from seeing the solder under it. So, it's also a good idea to keep an eye on the bottom of the joint. Once the joint is full, any extra solder you add will come dripping out the bottom of the joint. So, once you see a bulge of solder starting to form under the joint, that's a clear indication that the joint is full of solder as well.

Sometimes a joint will keep taking solder, and that means that the solder is puddling up inside the pipe. This can be a bit unnerving because by the time you realize what's happening, you've typically melted too much solder into the joint. So, it's a good idea to keep track of how much solder you're melting into each joint, and stop when you've used about the diameter of the pipe's worth. That is, for soldering a 1/2 inch pipe joint, you'd expect to use about 1/2 inch of solder; for a 3/4 inch pipe joint it's 3/4 inch of solder. If you don't see a silver ring form at the joint or the solder starting to form a drop under the joint by the time you've used a pipe diameter's worth of solder, then presume the solder is flowing into the pipe and stop.

Special cases to be aware of:

The most common causes of leaking solder joints are:

1. The joint not being hot enough all the way around it's circumference (which is why most people apply the heat on one side and the solder on the other). Water boiling off into steam absorbs heat like crazy, so if there's any water in the piping you're trying to solder you need to remove the water from the immediate area where you're soldering. If water is dripping into the area of the pipe you want to solder you can stop the water from getting near the joint you're trying to solder by stuffing bread into the pipe. You tear the brown outer crust off a piece of white bread, knead the bread into a plug and stuff that plug into the leaking pipe. That stops the leak for long enough to solder the joint, and the bread will dissolve in the water once the plumbing is put back in service.

Another reason for the joint not to be hot enough is because of the kind of torch you're using. You want to use a torch that mixes the propane with air before igniting it. That gives you a much hotter flame than a "pencil tip" torch which doesn't allow for that pre- mixing.

2. Another common cause of bad solder joints is people not allowing for air expansion inside the piping when soldering. When you heat the solder joint, the air inside the pipe will heat up and expand and create a small pressure inside the pipe. It's capillary pressure that draws solder into the joint, and so if there's any pressure inside the pipe then the capillary pressure drawing solder into the joint will have to overcome the internal pressure in the pipe trying to push that solder out. So, always try to provide a path for any air pressure in the pipe to escape OTHER than the joint you're trying to solder.

Special soldering cases:

When soldering a valve onto a copper pipe, it's generally best to take the valve apart and only solder the body of the valve in. When the body is cool, then you can put the valve back together. That way you avoid overheating any of the plastic or rubber parts in the valve. The exception to this rule is ball valves. You'll greatly increase the chances of having a leak if you take a ball valve apart before soldering it in. Ball valves are best soldered into place undisassembled and in the open position. If you solder the valve in the closed position, it's possible for the air pressure trapped inside the ball to deform the teflon seals so that the valve leaks afterwards. I normally take the handles off ball valves before soldering them to avoid burning any plastic on them.

You connect copper pipes together with a fitting called a "coupling". The most common kind of coupling to find available at your hardware stores are the slip coupling, dimple stop and rolled stop:

A slip coupling like this one doesn't have a "stop" to ensure that it straddles the joint between two pipes.

Dimpled stop and rolled stop couplings have a "stop" to ensure they straddle the middle of the joint between two pipes.

The problem is that the dimple stop couplings are probably the most common to find available because you can also use them as a slip coupling by hammering out the dimple.

The problem is that lots of people will tell you to FILE the dimple down from the ID of the coupling with a small round or half round file, and that's terrible advice.

The reason why is that the dimples are often as deep as the wall thickness of the coupling. So, if you file them off to slip the coupling over the pipe, the wall thickness where the dimple was can be almost non-existant.

If your local hardware store only stocks these dimple stop couplings, the best way to use them as slip couplings is to fit them over a socket and hammer the dimple out. That way you're not reducing the wall thickness of the coupling at the dimple.

Nestor. This is a DIY site and we are supposed to be helping people fix things and do it correctly. Telling members that you don't need to use flux, but can use other things when soldering is asking for problems for the members who believe you. I have explained in 7 sentences what you have not finished in 3 long posts. I have not seen any members ask what you are trying to explain. Sorry, but it looks like you have too much time on your hands. I have been doing this type work for 35 years.

Travelover:
I don't know much about the different kinds of solders cuz like Redwood says, I've always had to use the lead free solder in my building for the past 20 something years, so I've never had any experience with any other kind of solder.

Carrying on...

Anyhow, lots of people won't know how to handle a particular situation when they first start soldering. They may be under the impression that you always have to get back to bare copper before you can re-use a pipe end or socket that has solder on it. And, that's not true. The standard practice is that if the solder is sticking well to the copper, then you just treat the old solder like the new surface of the pipe or socket.

For example, if you unsolder a valve from the end of a pipe (like unsoldering a Brasscraft stop from a water supply pipe when renovating a bathroom. When you finish your renovations and you want to put a new Brasscraft stop on that same pipe, do you need to sand off all the old solder to get the pipe down to bare copper before soldering the new valve on?

The answer is no. That's cuz the old solder on the pipe is sticking to that copper as well as any new solder will, so why remove it? In that case, you'd simply treat the old solder as the new surface of the pipe. Sand the old solder lightly and flux it the way you would the pipe end. If that old solder prevents the new valve from slipping onto the pipe, then heat the pipe and wipe the excess solder off the pipe with a dry rag so that the new valve will fit on the pipe. Then just give the solder on the pipe a light sanding to remove any lead, tin or antimony oxide formed as the pipe was cooling down, flux the old solder on the pipe, brush out the socket on the new valve and flux it, and then just solder the valve onto the pipe end normally. You wouldn't need to sand the old pipe down to bare copper before soldering.

Or, for example, if you unsolder a pipe from the middle port of a tee fitting and want to solder a new pipe in, do you need to somehow get all of the old solder out of that socket (with a piece of sand cloth rolled around your finger, maybe)?

No, cuz that old solder is sticking as well to the copper as the new solder will, so why remove it? You can just scrub out the old socket with a fitting brush, apply flux to the inside of the socket, sand and flux the new pipe end and solder them together. However, you'll probably wreck the fitting brush doing that because you'll probably reduce the diameter of the brush and it won't brush out new sockets as agressively as it should.
A better way would be to sand and flux a small piece of scrap copper pipe, heat the tee to melt the solder in the socket and slide the sanded and fluxed pipe end into the socket. The old solder inside the socket will immediately bond to the pipe end, and then by pulling that pipe end out again (if the joint is still hot) you can remove the excess solder from the socket.
Then, once things cool down, treat the residual solder in the socket as the new surface of the fitting, and brush out the tee's socket and flux it just like it was copper. Sand and flux the pipe end, put them together and solder normally.

And if you're ever looking at a big drop of solder on the bottom of a pipe that's preventing you from fitting a socket over that pipe, but it's taking forever to sand ALL the old solder off, you should realize you don't need to sand ALL the old solder off. If the end of the pipe is sanded, just flux it, brush out the socket and flux it, then heat the pipe until the old solder melts and the socket slips over the pipe. Then just continue heating and add some solder just to confirm the joint is full.

In all of these cases, what's important is that you remove any oxide on the surface of the pipe end or socket and flux whether that oxide is a copper oxide on the surface of copper or lead, tin or antimony oxide on the surface of solder. The zinc chloride will remove what you didn't by sanding at soldering temperatures.

But, if you insist that you have to remove all of the old solder from a pipe and sand the pipe down to bare copper before the solder joint has been done "properly", then use a small flat file to remove blobs of old solder from copper piping. You can die of old age trying to SAND a drop of solder off a copper pipe. And, if it's old solder, you might be inhaling lead dust. A file scrapes the solder off in bigger chunks that drop to the ground rather than float in the air.

And, whenever you're going to be covering up your soldering with drywall, pressure test the plumbing for leaks first. I find one of the best ways to do this is to put a piece of paper towel under each potential leak. That way, if the joint leaks, it'll leave a dark mark on the paper towel which is much easier to spot than clear water on a non-absorbing surface like a metal stud or something. And, use paper towels with an embossed pattern pressed into them. That way, even if the leak stops for some reason, and the wet spot on the towel dries, you'll still see that there was a leak because when the paper towel gets wet the embossed pattern will relax and the wet area of the towel will dry flat.

Never ever never cover up all your soldering with drywall and joint compound and paint without pressure testing all your joints first. If they don't leak in the first coupla minutes, they won't leak in the next coupla centuries.